The present invention relates to liquid crystal optical valves controlled by photoconducting effect which may serve as image converters. These devices use a cell, subjected to an electric biassing voltage distributed between a thin layer of liquid crystal and a photoconducting plate joined to this layer. It concerns more particularly the display of images produced by X rays in radioscopy or radiography, and the projection of images in incoherent light as well as use in data processing devices using coherent radiation, (for example an optical correlator).
Such a cell is based on the following known effects: on the one hand, the property which mesomorphic materials have of modulating an optical radiation depending on an electric field which is applied to them and, on the other hand, the property of photoconducting materials whose electric resistance is reduced when receiving an incident light radiation.
Mesomorphic materials are made of elongated molecules that can be orientated in the presence of a solid wall, in a common direction which may be either parallel or perpendicular to the plane of the wall. The direction of this orientation depends on the respective natures of the liquid crystal material and of the wall. The orientation of the long molecules of the liquid crystal is further greatly facilitated by the introduction of traces of appropriate surfactants in the mesomorphic material as well as by previous treatment of the walls in contact with the film (rubbing the wall, evaporation under grazing incidence of a film of silicon oxide). Depending on the effect desired, a mesomorphic material will be used presenting one or other of the three phases: smectic, nematic or cholesteric. In devices using a photoconductor associated with a liquid crystal, generally a nematic type of liquid crystal is used. Here briefly are some of the different uses of such liquid crystals.
The long molecules which form mesomorphic materials present pronounced dielectric anisotropies: consequently, they tend to orientate themselves parallel or perpendicular to an electric field depending on whether this anisotropy is positive or negative. Thus, by gripping a plate of nematic material between two transparent electrodes subjected to a variable DC or AC electric voltage, it can be seen, if the material presents a positive dielectric anisotropy and is disposed in parallel orientation, that beyond a threshold voltage (of the order of 1 to 5 volts for a film of a thickness of 10 microns or so) that the birefringence of the plate measured perpendicularly to its plane decreases when the applied voltage is increased: the molecules tending to orientate themselves parallel to the field. Conversely, for a nematic material having a negative dielectric anisotropy subjected to the same field as before, the birefringence increases when the applied voltage is increased. This effect, called "controlled birefringence", if the voltage applied to a nematic film disposed between parallel or crossed polarizers can be varied locally, allows the intensity of monochromatic light transmitted by the plate or the color of polychromatic light to be modulated spatially. According to a known technique, it is also possible to use the dielectric and optical anisotropic properties of the nematic materials for electrically controlling the light transmitted by a thin layer by using the so-called "twisted nematic" structures. The intensity of the light transmitted by the layer will depend on the voltages applied locally to this layer.
Another way of controlling the light transmitted or reflected by a nematic liquid crystal cell is based on the "dynamic diffusion" phenomenon which appears beyond a second threshold voltage and which very quickly masks the birefringence phenomenon. Like controlled birefringence, dynamic diffusion is perfectly reversible. However, by adding to the nematic material a small percentage of cholesteric material, a mixture is obtained endowed with a memory. By application of an AC voltage at a frequency higher than the control voltage the mixture can be brought back to its initial transparent state.
In the patent application field in France on Sept. 23rd, 1977, under the national registration number 7 728 738 and entitled "Dispositif de visualisation comportant une couche mince de cristal liquide accolee a une lame photoconductrice", the Applicant has described a device using a nematic phase liquid crystal layer joined to a photoconducting material plate. To the cell formed by the association of liquid crystal and the photoconducting material is applied a bias voltage. In this device, the voltages applied to the cell may be very high all the more so since sensitivity of this cell to writing radiation increases when the value of the control voltage increases. In this case, the control voltage may reach 1000 V and approximate the breakdown voltage of the cell.
The present invention proposes reducing the biasing voltage to a much lower value by using an AC voltage whose frequency is chosen as a function of the dynamic potentiometric operation. Thus the sensitivity of the cell may be increased without fearing the breakdown voltage and the size and the price of the control voltage generator reduced. The reading radiation may pass through the valve or be reflected by a layer disposed for this purpose.